Growth plugs are small independent segments of stabilized media which can house a variety of biological objects. The plugs typically serve as a medium for plant material, e.g., seeds, seedlings or cuttings, that will eventually be transplanted into a new environment. In addition to serving as a growth medium, such plugs can be used for propagation, germination, rooting, micro propagation, clonal propagation, and a variety of in vitro techniques. Growth plugs are commonly used as an environment for seeds, seedlings or cuttings, often followed by transplantation.
Growth plugs usually consist of a block of growth medium, e.g., about 2 to 3 inches long and ½ to 1 inch in diameter. The growth medium may include soil, peat, mulch, thatch, sand or any other organic, inorganic or gel substrate, depending on the environment most suited to the cutting, seed or seedling. Typically, the plugs will house a cutting, seed or seedling; however, they can be fashioned to house larger growths, including saplings and immature shrubs.
The growth plugs provide a compact, economical and stable medium in which to house plant material for sale. Most decorative plants and flowers for home gardens are sold in growth plugs. The growth plugs are often housed in flats or trays constructed of polyethylene, polystyrene or poly(vinyl chloride) (PVC) plastic. This lightweight plastic has become the material of choice for filling flats and trays because it is inexpensive to manufacture and easy to ship and store. Each tray is typically designed as a block with, for example, four to six, tray cavities with tapered ends that hold the individual plugs. The bottom of each cavity is usually solid, with either a few small holes or one large hole to facilitate drainage from the housed plug. The sides of the cavity may be smooth or porous. Alternatively, open-ended cavities may be used, to increase airflow around the growth plugs, resulting in better growth. The plugs are compressed into the cavities, and, depending on the hardiness of the particular variety of plant, the flats may provide a sustainable environment for the cutting, seed or seedling for many weeks.
Growth plugs made of composite polymerized medium, such as the medium described in U.S. Pat. No. 6,322,734, issued to Zanten et al., are frequently used in growth trays as described above. Such polymerized growth media have many advantages over traditional media, such as soil and peat moss. For example, a polymerized growth plug holds its shape without the need for a pot to retain the medium, reducing the cost of manufacture and transport. Such polymerized media are typically mixed in a slurry, comprising a mixture of aggregate (including but not limited to peat, bark, coconut fiber, etc.), water, surfactant (e.g., lecithin, alkylsulfonates or phospholipids), and a polyurethane prepolymer, e.g., a hydrophilic polyurethane prepolymer. When the prepolymer is mixed with the other components, the prepolymer begins to polymerize, generating carbon dioxide. The carbon dioxide causes the slurry to rise, not unlike a cake rises from gas created by yeast, and the polymerization of the prepolymer stabilizes the resultant substance, not unlike the effect of egg in a cake. The reaction time is temperature or pH dependent or both; for example, the higher the temperature, the shorter the time it takes the mixture to rise and polymerize. In a typical reaction at 70 F, the mixture begins generating carbon dioxide within approximately five to ten seconds after mixing, with full rise within one to two minutes and a stable growth plug within approximately six minutes.
Various methods of manufacturing growth plugs are known in the art. U.S. Pat. No. 6,322,734, issued to Zanten et al., describes composition of polymerized growth plugs and a method for continuous filling of trays. The components of the polymerized growth medium are combined in a mixer and then emptied into trays as they pass beneath the mixer by means of a conveyor belt. A weighing device measures an appropriate amount of slurry for the tray being used.
U.S. Pat. No. 5,209,014, issued to Teichmann, discloses the use of a “mold tool” which is filled with composite growth medium, creating a growth plug or “mold” into which seeds may be planted. The molds are filled individually, rather than in a tray or array configuration. Using a rotating table, the molds are passed through various stations in a machine, at which stations components of the composite are added.
U.S. Pat. No. 4,175,355, issued to Dedolph, discloses a method of forming polymerized growth plugs individually in cylindrical molds. The molds are sprayed with a nonphytotoxic release agent, then individually filled from a nozzle with polymerized growth medium in a slurry form.
U.S. Pat. No. 4,159,727, issued to Visser, discloses a machine for filling a plurality of individual pots with soil from a hopper as the pots pass underneath the hopper on an endless-chain type conveyor. Excess soil which spills over the tops of the pots is pushed onto an elevator and returned to the hopper.
Also known are methods for filling trays containing multiple tray cavities with various thick fluid substances. U.S. Pat. No. 4,747,766, issued to Van Meulenbeke, is directed to a machine for filling trays with chocolate or another thick fluid substance. It discloses a machine in which trays containing an array of tray cavities pass on a conveyor belt beneath a tank containing the thick fluid substance. The tank features apertures in its underside that line up with the cavities, allowing the simultaneous filling of an entire row of tray cavities.
Remarkably, the present invention provides a machine and methods to mix the slurry and deposit it uniformly and quickly into trays, distributing it evenly over multiple tray cavities, such that growth plugs of a desired size, shape, and density may be rapidly and easily produced, before the slurry cures into a firm growth plug. Notably, the machine and methods should provide the flexibility to accommodate trays and tray cavities of a variety of sizes, as well as the ability to create growth plugs with different densities to suit the requirements of a wide variety of plant species.